linux-kernel - RE: [tip:irq/core] genirq/matrix: Improve target CPU selection for managed interrupts.

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Date:   Wed, 7 Nov 2018 18:41:57 +0000
From:   Michael Kelley <mikelley@...rosoft.com>
To:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "mingo@...nel.org" <mingo@...nel.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "hpa@...or.com" <hpa@...or.com>,
        Michael Kelley <mikelley@...rosoft.com>,
        Long Li <longli@...rosoft.com>
Subject: RE: [tip:irq/core] genirq/matrix: Improve target CPU selection for
 managed interrupts.

From: tip tree robot <tipbot@...or.com>  Sent: Tuesday, November 6, 2018 2:28 PM
>
> Committer:  Thomas Gleixner <tglx@...utronix.de>
> CommitDate: Tue, 6 Nov 2018 23:20:13 +0100
> 
>  2) Managed interrupts:
> 
>     Managed interrupts guarantee vector reservation when the MSI/MSI-X
>     functionality of a device is enabled, which is achieved by reserving
>     vectors in the bitmaps of the possible target CPUs. This reservation
>     decrements the available count on each possible target CPU.
>

Thomas,

For the curious, could you elaborate on the reservation guarantee for
managed interrupts?  What exactly is guaranteed?  I'm trying to
understand the benefit of reserving a vector on all possible target CPUs.
I can imagine this may be to related hot-remove of CPUs, but I'm not
seeing the scenario where reserving on all possible target CPUs solves
any fundamental problem.  irq_build_affinity_masks() assigns spreads
target CPUs across each IRQ in the batch, so you might get a small handful
of possible target CPUs for each IRQ.  But if those small handful of CPUs
were to be hot-removed, then all the reserved vectors disappear anyway.
So maybe there's another scenario I'm missing.

Thanks,

Michael